In currently used optical transmission systems, optical signals are converted to electronic ones before processing occurs. Such processing involves the use of standard electronic devices. In the next generation of optical communication systems, it is envisioned that optical signals will be processed without conversion to electronic signals. Such optical processing will require optical devices which are analogous to devices such as amplifiers, modulators, filters, multiplexers, demultiplexers, for example, which are used for processing electronic signals.
A tunable optical filter having a bandwidth between about 100 MHz and a few tens of gigahertz with low insertion loss and being easily manufacturable would be an important component in wavelength multiplexing as well as in many other applications. It appears that the most promising approach to such a device is a fiber Fabry-Perot interferometer which may be referred to as an FFP.
A Fabry-Perot interferometer is an optical device which can be used to process optical signals and includes two mirrors with a cavity therebetween. The Fabry-Perot interferometer is discussed, for example, in Born and Wolf, Principles of Optics, MacMillan, 1959, pages 322-332. An exemplary Fabry-Perot structure comprises a region bounded by two plane, parallel mirrors. The structure exhibits low loss, that is, it passes only particular wavelengths for which the cavity is said to be in resonance--a condition obtained by adjusting appropriately the cavity parameters. At resonance, the cavity passes a series of equally spaced wavelengths. The spacing between these wavelengths, called the free spectral range or tuning range of the cavity (FSR), is a function of the spacing between the mirrors and the index of refraction of the medium between the mirrors. The tuning range of a Fabry-Perot interferometer is equal to c/2nl.sub.c where l.sub.c is used to designate the length of the cavity. Accordingly, the shorter the cavity, the larger the tuning range. The bandwidth is largely determined by the reflectivity of the mirrors; however, other sources of loss and reflections can affect bandwidth. Another parameter which is designated finesse is equal to the quotient of the tuning range divided by the bandwidth.
The use of Fabry-Perot cavities as filters in commercial optical fiber communication systems to process optical signals is known. However, the use of such devices has been hampered by, among other constraints, the lack of practical designs which have suitable characteristics, such as low loss when used with optical fibers, appropriate values of free spectral range and suitable means for tuning the devices and maintaining alignment of the optical fiber end portions.
Designs that more closely meet the needs of a commercial fiber system have been suggested. In Electronics Letters, Vol. 21, 76.11, pp. 504-505 (May 12, 1985), J. Stone discussed a fiber Fabry-Perot interferometer design in which the cavity comprised an optical fiber waveguide segment with mirrored ends. The free spectral range of the resulting cavity is determined by the length of the fiber segment. The cavity can be tuned over one free spectral range by changing the cavity optical length by one-half the wavelength value of the light entering the cavity. In this way, the cavity can be tuned to resonate at, and therefore transmit light of different wavelength values. To obtain such tuning, the cavity length can be changed, for example, by means of a piezoelectric element attached to the fiber, which, when activated, will stretch the fiber and increase the associated cavity optical length accordingly. Fiber Fabry-Perot interferometers can be made with a finesse up to 500 with relatively low insertion loss, using separately attached mirrors.
In an article entitled "Pigtailed High-Finesse Tunable Fiber Fabry-Perot Interferometers With Large, Medium and Small Free Spectral Ranges", authored by J. Stone and L. W. Stulz, appearing in the July 16, 1987 issue of Electronics Letters beginning at page 781, the authors disclosed that fiber Fabry-Perot devices with any required bandwidths can be fabricated from one of three types of structures, Types 1, 2 and 3, reported in that article.
A Type 1 structure reported by Stone and Stulz is a fiber resonator. Mirrors are deposited on both ends of a continuous fiber and tuning is achieved by changing the optical length of the fiber. This type of fiber Fabry-Perot interferometer generally is limited to a length greater than 1 to 2 cm which equates to a free spectral range on the order of 10 to 5 GH.sub.z. Although no alignment is required, the bandwidth range is limited to less than 100 MHz for a finesse of 100 and an l.sub.c of 1 cm.
Among the advantages of the Type 1 fiber Fabry-Perot interferometer is the fact that the cavity comprises an optical fiber which is a waveguide. This eliminates deleterious diffraction effects present in long Fabry-Perot cavities which are not waveguides. The elimination of the deleterious diffraction effects is associated with the guiding characteristics of the fiber. However, the difficulty of working with and stretching small lengths of optical fiber precludes large values of free spectral range when using a Type 1 fiber Fabry-Perot. As a result, the usefulness of the Type 1 fiber Fabry-Perot design is somewhat limited.
A Type 2 fiber Fabry-Perot interferometer is a gap resonator with mirrors deposited on adjacent end faces of two optical fibers. In this type of filter, the diffraction loss between the fibers limits the resonator gap to less than 10 .mu.m which corresponds to a free spectral range greater than 10,000 GH.sub.2 or approximately 750 .ANG..
Large free spectral ranges can be obtained by using a Type 2 fiber Fabry-Perot interferometer in which the cavity comprises a small gap. However, because of diffraction losses, larger gap cavities are less practical, and therefore the Type 2 Fabry-Perot interferometer is not adequate for applications which require the smaller free spectral ranges otherwise associated with larger gaps. Unacceptable losses result from gaps in excess of 10 .mu.m.
A Type 3 structure is an internal waveguide resonator. A mirror film is applied to an end of one external fiber disposed in the passageway of a ferrule and another to one end of an internal waveguide. The ferrule which supports the external fiber is movably mounted in a sleeve in which also is disposed the internal waveguide and another ferrule in which an optical fiber is disposed. A relatively small gap separates the mirrored end of the external waveguide and an unmirrored end of the internal waveguide. Scanning is accomplished by changing the spacing of the small gap between the mirror film at the end of the external fiber and the internal waveguide. The free spectral length is determined by the length of the internal waveguide which can be made in lengths as short as 1 mm or less. An anti-reflection coating may be applied to the non-mirrored end of the internal waveguide. Although the Type 3 fiber Fabry-Perot interferometer covers the most practical range of frequencies, it may be somewhat difficult to manufacture because of the lengths of the internal waveguide.
In each of the above-described three types of Fabry-Perot interferometers, the fiber ends are disposed in standard glass or ceramic ST.RTM. connector ferrules. Afterwards, the ends are polished and coated with multi-layer dielectric mirrors. The ferrules are held in alignment with either a split or solid zirconia sleeve and the assembly is mounted in a piezoelectric shell which is attached to the ferrules. Should a fiber connection be needed, it may be carried out by connecting ST or rotary splice connectors to the outer ferrule ends for the Type 1 or to fiber pigtails for Types 2 or 3.
In commonly assigned application Ser. No. 07/466,536 which was filed on Jan. 17, 1990 in the names of J. B. Clayton and C. M. Miller, a resonant cavity approach was used to obtain desired sharp filtering. In order to obtain a desired tuning range and bandwidth, the cavity length may range between a few microns and several millimeters.
The filter of the above-identified application Ser. No. 07/466,536 comprises first and second ferrule assemblies each having aligned passageway portions in which is disposed optical fiber. The passageway portions of each ferrule assembly are spaced apart by a mirror which is normal to a longitudinal axis of the passageway portions and which is closer to one end of the ferrule assembly than to an opposite end thereof. The first and second ferrule assemblies are held with the axes of the passageways aligned and with the one end of said first ferrule assembly being adjacent the one end of said second ferrule assembly and with the adjacent one ends of the ferrule assemblies having a predetermined axial spacing. The mirrors of the ferrule assemblies extend over only a portion of the transverse cross-sectional area of an associated ferrule.
In the preparation of the filter just described, two ferrules each having a passageway therethrough with optical fiber disposed therein and one of which has a mirror formed on one of the end faces thereof are aligned, using an active alignment process by measuring power, and then are bonded together with the mirror therebetween. Then one of the ferrules is severed to provide a wafer having a newly formed end surface which is polished. Two such wafered ferrules which are referred to as ferrule assemblies are positioned with the passageways aligned and with the mirrors being adjacent to each other. With such a construction, only a few percent of the light at a non-resonant wavelength is passed through each mirror. All reflections in the resonant cavity add in phase and a relatively low throughput loss is achieved. By changing the cavity length, that is, the distance between the mirrors, tuning capability is provided.
The problem is to obtain a very sharp narrow band optical filter with cavity lengths tunable from a few microns to several millimeters which correspond to bandwidths between a few tens of gigahertz and approximately 100 MH.sub.z with a stable repeatable design that is relatively easy to manufacture with high yield. One can appreciate the complexity of the problem when using single mode optical fiber. There with a core diameter of 8 microns or less, at a finesse of 100, the light beams propagate back and forth approximately 100 times before passing through the resonant cavity. As a result, the arrangement must be 100 times more sensitive to alignment than single mode optical fiber.
The problem is that of providing a fiber Fabry-Perot interferometer with the capability of adjusting the gap between the exposed end faces of the wafers to tune the interferometer. Also, the sought after device is an optical filter which includes facilities for adjusting the alignment of the fiber end portions, particularly in view of the longitudinal relative movement therebetween, in order to achieve low loss. Still further, the sought after optical filter has a relatively high extinction or contrast ratio, that is, one which has a large difference between the passband and the stopband insertion loss.